lOO THE BIOLOGY OF TWINS 



a potent male. This interpretation is evidently due to 

 some extent to the preconceived idea that the twins are 

 monozygotic and therefore should be of the same sex. 

 It would seem unlikely that twins of opposite sex are 

 ever derived from a single fertilized egg. Such a 

 finding would be totally out of accord with what we 

 know of the chromosomal basis of sex-determination in 

 mammals. Yet such a view has not been without 

 adherents. Bateson, for example, in his Problems of 

 Genetics makes the following statement about free- 

 martins : 



In horned cattle twin births are rare, and when types of 

 twins of opposite sexes are born, the male is perfect and normal, 

 but the reproductive organs of the female [italics mine] are 

 deformed and sterile, being known as a freemartin. The same 

 thing occasionally occurs in sheep, suggesting that in sheep also 

 twins may be formed by the division of one ovum [italics mine]; 

 for it is impossible to suppose that mere development in juxta- 

 position can produce a change of this character. I mention the 

 freemartin because it raises a question of absorbing interest. It 

 is conceivable that we should interpret it by reference to the 

 phenomenon of gynandromorphism, seen occasionally in insects, 

 and also in birds as a great rarity. In the gynandromorph one 

 side of the body is male, the other female. In such cases neither 

 side is sexually perfect. If the halves of such a gynandromorph 

 came apart, perhaps one would be a freemartin. 



This statement commits Bateson to the theory of 

 monozygotic origin of heterosexual cattle and sheep 

 twins and to the interpretation of the freemartin as 

 a sterile female. 



Recently Cole^ in a brief abstract takes a view of 

 the value of the freemartin quite in accord with that 



^ L. J. Cole, Science, N.S., XLIII (1916). 



